Process for electrolytically applying polymer coatings on electroconductive articles

ABSTRACT

HYDROPHOBIC POLYMER COATINGS ARE APPLIED TO SOLID ELECTROCONDUCTIVE ARTICLES BY SUBJECTING A POLYSULFONIUM SALT IN SOLUTION IN AN ELECTROLYSIS SOLVENT TO AN ELECTRICAL POTENTIAL SUFFICIENT TO REDUCE THE SULFONIUM SALT. THE SOLID ELECTROCONDUCTIVE ARTICLES ARE USED AS THE CATHODE IN THE ELECTROLYSIS PROCESS. IN EXAMPLE, A LEAD ELECTRODE IS COATED WITH POLY(P-XYLYLENE) BY ELECTROLYZING AN AQUEOUS SOLUTION OF (P-PHENYLENEDIMETHYLENE)BIS(DIMETHYLSULFONIUM CHLORIDE).

United States Patent ()ffice 3,697,398 Patented Oct. 10, 1972 U.S. Cl.204-14 N 9 Claims ABSTRACT OF THE DISCLOSURE Hydrophobic polymercoatings are applied to solid electroconductive articles by subjecting apolysulfonium salt in solution in an electrolysis solvent to anelectrical potential suflicient to reduce the sulfonium salt. The solidelectroconductive articles are used as the cathode in the electrolysisprocess. In example, a lead electrode is coated with poly(p-xylylene) byelectrolyzing an aqueous solution of(p-phenylenedimethylene)bis(dimethylsulfonium chloride).

CROSS-REFERENCE TO RELATED APPLICATIONS This application is acontinuation of our U.S. patent application S.N. 879,510, filed Nov. 24,1969 (now abandoned), which in turn is a continuation-in-part of ourU.S. patent applications S.N. 647,895 and 647,896, both filed June 22,1967. The latter applications are now U.S. Pat. 3,480,525 and U.S. Pat.3,480,527.

BACKGROUND OF THE INVENTION A process for preparing poly(p-xylylene) byelectrochemical reduction of sulfonium salts, such as u,ot-biS(dimethylsulfonium chloride)-p-xylene, in various electrolysis solvents, such aswater, was described in our copending U.S. patent application, Ser. No.647,895. The process comprised subjecting a solution of said sulfoniumsalts to an electrical potential sufficient to reduce the sulfoniumsalt. The poly(p-xylylene) thus prepared coated various metalliccathodes in the electrolysis process, namely aluminum, tin and lead. Theprocess was operable at both controlled and uncontrolled electricalpotentials, i.e., the cathode potential is, respectively, maintained ata constant value (volts), or allowed to vary from the initial cathodepotential towards a more negative voltage as the cathode is coated andthe current ceases to flow in the electrolysis cell.

The polarography of sulfonium salts is known, as described by Colichmanand Love, J. Org. Chem., 18, 40 (1953).. They used a stirred mercurycathode and a very carefully monitored cathode potential. However, thecoating phenomena on solid electroconductive materials was unknown untilour discovery thereof.

The subject invention pertains to an electrochemical reduction oforganic polysulfonium salts to form protective, decorative and otherwiseuseful coatings on solid electroconductive articles. Said reductionrequires an electrolysis system comprising an anode, a cathode, anelectrolysis solvent and a means for establishing and maintaining anelectrical potential between said anode and cathode.

SUMMARY OF THE INVENTION It has now been discovered that solidelectroconductive articles can be easily coated with a polymericmaterial by using such electroconductive articles as the cathode in theabove described electrolysis system and subjecting a polysulfonium saltin solution in the electrolysis solvent to an electrical potentialsuflicient to reduce the polysulfonium salt.

The importance of this invention resides in the fact that the solublepolysulfonium salts can be electrolytically converted to insolublepolymers on the surface of many articles regardless of their size orshape. The subject coatings are generally obtained as a thin film whichadheres to the cathode surface. These coatings can and do impartdesirable properties to the coated substrate, such as electricalinsulation for wiring, corrosion, and oxidation protection for metalparts, abrasion-resistant surfaces for working parts subject to wear,and so on. Decorative or colored coatings can also be applied inaccordance with this invention, e.g., an article may be obtained havingan orange-peel effect on the surface, or a high gloss or color may beobtained (the color resulting from a chromophore( s) in the polymerstructure). The physical properties of the coating will vary dependingupon the particular polysulfonium salt(s) chosen, but, the coatings aregenerally infusible and hydrophobic, particularly when the coatedcathode is subsequently heated to a temperature which thermallydecomposes any residual sulfonium groups and evaporates any entrainedsolvent.

Suitable polysulfonium salts in this reaction may range in molecularweight from monomers to high polymers.

The lower molecular weight salts can be electroreductively coupledduring the deposition process to give higher molecular weight products.These products can then be converted to even higher molecular weight orcrosslinked products by subsequent curing.

The higher molecular weight salts on the other hand need only to beinsolubilized at the cathode surface by electrolytically destroying someor all of the sulfonium moieties responsible for its solubility. Theproperties of such a coating may be further enhanced by an increase inmolecular weight pursuant to chain-extending or crosslinking arising asa consequence of a reductive coupling reactlon.

The polysulfonium salts suitable in the subject invention aresubstantially soluble in the electrolysis solvent and have the generalstructural formula I [nim x-na wherein R and R are hydrocarbon orhydroxy-substituted hydrocarbon groups of 1 to about 30 carbon atoms; nis an integer and is at least 2; X is an n-valent hydrocarbon radicaland is preferably an activating hydrocarbon radical (i.e., a radicalwhich causes preferential cleavage of the +S-X bond) whose chain lengthmay be interrupted by oxygen, sulfur or nitrogen, or by a keto, ester oramide linkage; and A is an electrolytically acceptable anion, and istypically the anion of an acid, such as hydroxide, chloride, bromide,nitrate, sulfate, bicarbonate, phosphate, acetate, maleate, benzoate andthe like.

X in (I) above includes: polymethylene (aryl) radicals, Such asC5H4(CH2)2, C5H (CH2)3,

CH2C6H4CH2CH2-C3H4CH3, POIYSUlfOIlluIIlS prepared from halo'methylatedpolystyrene or a-methylstyrene and the like; polymethylene (polyaryleneoxides and sulfides), such as the polysulfoniums prepared by reacting adialkyl sulfide, e.g., dimethyl sulfide, with di-, triand/ortetra-chloromethylated diphenyl oxide or sulfide polysulfoniums fromhalomethylated polyphenylene oxide, and the like; polymethylene(polyarylene carbonates), such as am F r 9 (Paola? polyradicals derivedfrom polymers and copolymers of 2-chloromethyl-1,3-butadiene; and otherlike groups having 2 or more activated S--X bonds. An activated radicaltherefore includes those groups having an aromatic, olefinic or acylradical attached to a methylene carbon which is in turn attached to thesulfonium sulfur atom.

X in (I) above also includes non-activated hydro carbon radicals, suchas alkylene or arylene disulfoniums, and hydrocarbons having terminaland/or pendant polysulfonium groups, such as polyalkylenepolysulfoniums;e.g., polyethylene, polyvinyl chloride, polystyrene, and the like,having pendant q being an integer of from 1 to 100 or more;polymethylene (aromatic polyesters and polyamides), such as the Q/ 89/---l polysulfoniums prepared by reacting a dialkyl sulfide with 2 1 thechloromethylated polyester or polyamide of terephthalic acid andethylene glycol or hexamethylenediamine, and the like; aromatic ketoneradicals, such as 0 -CH2 eH4 s 4 2 and the like; and other activatingradicals, such as: polyallylic and substituted allylic radicals, i.e.,polymers having pendant and/or terminal minal and/ or pendant and otherlike groups.'Other non-activated radicals include the polysulfoniumsmade from polyalkylene sulfides and polyarylene sulfides by reactingthem with methyl iodide, benzyl chloride, etc. Preferred polysulfoniumsalts are water-soluble salts including those of Formula I wherein: Rand R are hydrocarbon or hydroxy-substituted hydrocarbon groups of 1 toabout 10 carbon atoms, and most preferably 1 to about 4 carbon atoms; Xis polymethylene(aryl) or substituted polymethylene(aryl),polymethylene(polyarylene oxide or sulfide), particularly those of theformula --CH CH H CH CH C H --Y-C H CH wherein Y is oxygen or sulfur oralkylene, arylene or alkenylene of 1 to about 10 carbon atoms, andpolysulfonium salts obtained by the reaction of chloromethylatedpolystyrene with-sulfides 9/ and polysulfonium salts obtained by thereaction of copolymers of vinylbenzyl chloride and other vinyls orvinylidene, acrylic, methacrylate, maleatemonomers with groups;polyradicals derived by removal of chlorine atoms sulfides. Furtherrepresentative examples of suitable such from polymers and copolymers ofvinyl'benzyl chloride;

polysulfonium salts include those of Formula I wherein:

TABLE I R1 R: X n A- CH: CH3 -"CH7CH1 2 Cl- C;H|--. CzHs 2 Bl.

canon enema 2 r.

CH; CH: CHzCaH4-CHz- 2 N03. n-C H D-C Hl CH CcH -C|H4CH1 2 Tosylate CH101H: 3 Cl.

-CH;C H -CH1 n-(CHfisCILOI-I- n-(OHflsCHgOH 2 B1- CH: CHg

Il-ClflHfl n-CmHn -CH1C|H4OCsH4CH:- 2 F. CgH OH g 4CH:-C|H3Cl-0CgH3C1-CH3 2 OH.

H: a CHzC|H4(CH:)zCsH4CHr- 2 Tosylate.

C3H OH CQH'OH 3 or 4 C1.

CHI (CH2 I'-R!)I 0:1

n-CnHu -Cu a1 2 Tosylate. -C i- -S- -CH;-

n-CIHu n-CnHn 2 D0.

TABLE I-Continued 2. Nos.

C s.-..-...-............... C1117 C: a.-. CzHa 2...... Tosylate.

C ZC

n-C1uHz|-................... n-Cm n Il-CaHn.-................... l1-C Ho2...... Tosylate.

O 1a--......-.........-.. n-CnHu -CHC|H -CH CH2CH l j: n no I CH=CH3CH3............... CH3

C;H5..--...-..-.........-. CqH5 TABLE I-Continued R1 R: X 13 A" CH3 CH1R1 NO:-

CH -S R: L CH: Jzumnooo -@-oom o -s I OH OH n=1 to I C H:

stances such as plastics coated with aluminum and other like solidelectroconductive materials. Metals are the preferred classofelectroconductive materials, particularly lead, copper or iron alloysthereof, and chromiumor nickel-plated iron oriron alloys.

The subject in'ventionis surprisingly operable in aqueous media onmetals having a low hydrogen overvoltage,

suchas iron, nickel and copper, as well as on metals having a highhydrogen overvoltage, such as lead, zinc and tin. This is a tremendousadvantage since (a) most of the commercial items requiring a protectiveor decorative coating are metals having a low hydrogen overvoltage, and(b) water is by far. the preferred solvent based on availability, cost,lack of toxicity, etc.

Suitable electrolysis solvents, in addition to water, in-- clude inertorganic polar solvents, such as dimethylformamide, vacetonitrile,dimethylsulfoxide, hexamethylphosphoramide, tetrahydrofuran, loweral-kanols, such as methanol, ethanol, isopropanol, n-butanol, and thelike, and mixturesof such organic solvents, and mixtures of such organicsolvents with Water, and the like. The preferred solvents are thosewhich dissolve or solvate the polysulfonium salt and are inert to thepolymer coating, i.e., the solvent does not dissolve the coating to anysubstantial degree. Water and water-lower alkanol mixtures are thereforethe preferred solvents.

The process may be suitably conducted at controlled or uncontrolledelectrical potentials, as defined above; so long as sufficientelectrical energy is present to attract the polysulfonium to the cathodesurface and to reduce some or all of the sulfonium ions.

Since reductive coupling is essential to obtain the more useful polymercoatings from the lower molecular weight reactants, .the process must beoperated under conditions which. maximize the reductive couplingreaction. One method of achieving this is to use thecontrolled-potential electrolysis, as described by L. Meites inTechnique of Organic-Chemistry, A. Weissberger, editor, vol. 1, 3rd ed.,p. 3281, Interscience, N.Y. (1959). V

.For the higher molecular weight salts, an uncontrolled potential" isadvantageous, i.e., the driving potential is constant and thecathodepotential is uncontrolled." Polymers by their random nature and bulk arethought to physically prevent some of the attached sulfonium groupsfrom. contacting the cathode and therefore prevent complete sulfoniumreduction.

To some extent, the thickness of the coating can be increased, ifdesired, by including an optional supporting electrolyte. Any of theconventional supporting electrolytes may be employed, such electrolytestypically being salts of strong acids, such as KCl, NaBr, Na SO etc.Generally, a supporting electrolyte is not required and is preferablynot included.

The reaction temperature may be varied from the freezing point to theboiling point of the electrolyte solution (polysulfonium salt andelectrolysis solvent) so long as the applied coating is not dissolvedand the sulfonium reactant is not thermally decomposed. Typically, theprocess is con ducted at a temperature between about 20 C. and about 75C., although higher and lower temperatures may be used. Elevatedtemperatures are in some instances instrumental in increasing thecoating rate.

The electrolyte solution is preferably stirred, to assure a continuoussupply of sulfonium ions at the cathode surface. Violent agitation whichmight introduce air bubbles, etc., should be avoided.

The subject process may be suitably conducted as a batch or continuousprocess and the cathode may be only one member or a plurality of joinedmembers.

The process may be carried out from pH-3 to 12 but preferably from 7 to12, basic media is preferred to reduce the problem of gassing at theelectrode surface.

Preparation and coating of poly(p-xylylene) on an aluminum cathode pPhenylenedimethylene bis (dimethylsulfonium perchlorate) was preparedfrom the corresponding bis- (dimethylsulfonium chloride) by addingperchloric acid to an aqueous solution of the sulfonium salt. Theperchlorate precipitated and was isolated by filtration.

Four grams of the perchlorate thus prepared were dissolved in 200 ml. ofdimethylformamide (DMF) and placed in the cathode compartment of athree-compartment electrolysis cell. The anode and central compartmentswere filled DMF. An aluminum plate and a graphite rod were used as thecathode and anode, respectively. The catholyte solution was stirredduring the, electrolysis. The cathode potential (reducing potential) wasincreased until a current began to how (-10-15 ma.) and was maintainedat that voltage until suflicient current was passed to develop a coatingon the cathode. This coating was sequentially washed with water,toluene, DMF and acetone. The white coating was scraped from the cathodeand identified by infrared spectroscopy as poly(p-xylylene).

In identical experiments, except for the material used as the cathode,the following metal cathodes were similarly coated: tin, annealed steeland lead. The coated articles are resistant to oxidation in air and inwater and are protected against attack by aqueous HCl. The coating inthese cases was identified as poly-p-xylylene by contact infraredperformed directly on the coated metals.

EXAMPLE 2 Preparation and coating of poly(p-xylylene) on an aluminumcathode 0.3 mole of xylylene dichloride, 0.62 mole of bis-(hydroxyethyl) sulfide, and 450 ml. of deionized water were placed in astirred vessel for days at 50 C. The resulting solution was diluted to0.5 N in chloride with deionized water. The xylylenebis(diethanolsulfonium) chloride was then converted to the nitrate byadding AgNO until no more precipitate could be detected forming in theclear supernatant solution. The supernatant liquid was then filtered toremove AgCl. Crystals of p-xylylene-bis(diethanolsulfonium nitrate) wereformed when ethyl acetate and methanol were added to the water solution.The nitrate was dissolved (2 grams/ 80 cc. of solvent) indimethylformamide in which the electrolysis was carried out. Thecatholyte solution thus prepared was placed in an open beaker cellequipped with an aluminum cathode and platinum anode.

As voltage was applied, the current moved to about 60 ma./square inch ofaluminum but quickly decreased to about 20 ma./ square inch. A whitematerial coated the cathode in approximately 60 seconds. The coating waswashed with acetone and water and identified by infrared analysis aspoly-p-xylylene.

EXAMPLE 3 Preparation and coating of poly(p-xylylene) on a lead cathodeA 0.5 N aqueous solution of p-phenylenedimethylenebis(dimethylsulfoniumchloride) was made in KI and placed in a beaker equipped with a leadcathode and graphite anode. An electrical potential between the cathodeand anode (driving potentialfrom constant voltage source) of about 3volts was applied. The initial current was 50 milliamperes (ma.) whichdecayed to about 20 ma. in -20 seconds. A cloudy coating was easily seenon the cathode surface which had been machined to a shiny finish. Thecoating was identified as poly(p-xylylene).

EXAMPLE 4 Preparation and coating of poly(p-xylylene) on an aluminumcathode A 0.1 N aqueous solution ofp-phenylenedimethylenebis(dimethylsulfonium chloride (and in NaOH wasplaced in a beaker equipped with an aluminum cathode and graphite anode.Enough current was passed to form a coating on the cathode; the coatingwas identified as poly(p- Xylyleney EXAMPLE 5 A polyelectrolyte preparedby polymerization of vinylbenzyldimethylsulfonium chloride was ionexchanged with Dowex 1 resin to the polysulfonium hydroxide. The viscousorange aqueous solution (ca. 5% solids by weight) was placed in a beakerequipped with a graphite anode and a steel cathode (cathode surface areawas about 1 sq. in.). A potential was applied and raised to the pointwhere ma. of current was flowing. Very little hydrogen evolution(gassing) was observed. The current decayed to about 10 8 ma. during the1 hour of reaction. The cathode was removed and thoroughly rinsed withwater.

A blank was prepared by dipping a second steel chip into an identicalpolysulfonium hydroxide solution and then washing the chip with water.The blank and the test sample were dried in a hot air stream and placedin a 0.1 N aqueous NaCl solution at C. for 1 hour. The blank and theuncoated portion of the test sample turned black while the coatedportion of the test sample was unstained.

EXAMPLE 6 A polyelectrolyte was prepared by copolymerizing vinylbenzylchloride and butyl acrylate in an aqueous emulsion, diluting the mixtureto about 10% solids by weight and further reacting the copolymer with astoichiometric excess of dimethyl sulfide to produce the correspondingpolysulfonium salt (polyelectrolyte) which dissolved in the aqueousmedium. More water was added to make the concentration 4% by weight.

The solution was dialyzed against deionized water until free of solublesalts and the like. The resulting solution was slightly hazy andcontained 0.4% solids by weight. A portion of this solution was placedin a beaker containing a stainless steel anode and a copper wirecathode. A potential of 10 volts was applied. The initial current of5,00 ma. fell immediately to less than 10 ma. The cathode was removedand washed with water. The copper wire had a thin translucent gelcoating which cured when heated in air to a hard, continuous, insulatingand adherent film.

EXAMPLE 7 An aqueous solution of poly(p-xylene-a-diethylsulfowasdialyzed against deionized water until free of dissolved salts. Thesolution was diluted with water to 0.1% solids by weight. The solutionwas placed in a glass beaker containing a steel anode. (a) A steelcathode was placed in the beaker and a potential of 10 volts was appliedand the current rose to ma. A gel was rapidly deposited on the cathode.The cathode was removed and the coating dried (yellow coatings); (b) (a)was repeated at a higher potential and a 500 ma. initial current. Ayellow coating was immediately deposited and the current decayed to lessthan 10 ma. The cathode was removed, washed with water and heated in air(ca. C.). The resulting cured coating was a hard, continuous andinsulating coating. EXAMPLE 8 An aqueous solution (6% solids by weight)of a polyelectrolyte was prepared by reacting an excess of dimethylsulfide with a highly branched polymer prepared from dichloromethylateddiphenyl oxide; the polyelectrolyte had a wide and uneven molecularweight distribution with a major portion at about 190,000 molecularweight.

The polymer is represented by the recurring unit [G r i l]...

1 1 ma: The current decayed to 200 ma. in l min. and good polymercoating was obtained. Similar results were obtained with a zinc cathode(current decayed to 120 ma. in 3 min.) and a light'coating was obtainedon an aluminum cathode.

The initial solution was diluted with dimethylformamide to 3% solids byweight and made 0.3 N in tetraethylammonium tosylate. Repeating (b)above using this solution and a copper cathode, a good coating wasobtained with no substantial reduction in current.

EXAMPLE 9 An aqueous solution of a polyelectrolyte prepared by reactingan excess of dimethyl sulfide with. chloromethylated polystyrene (3.510- moles of C19/ gm. of dry polymer) in water. The solution was dilutedto 6% solids by weight and placed in a beaker equipped with a graphiteanode and a lead cathode. A potential was applied such that an initialcurrent of 500 ma. was achieved. The current decayed to 100 ma. in 3min. A good coating was obtained. Similar results were achieved using asteel cathode (no substantial decay in the amperage was observed,however).

In general, the polymer coatings are useful as electrical insulators, asunusual finishes, as colored coatings and other like uses.

We claim:

1. A process for applying a polymer coating to a solid electroconductivearticle used as the cathode in an aqueous electrolysis. systemcomprising an anode, a cathode, an electrolysis solvent and a means forapplying and maintaining an electrical potential between said anode andcathode, said process comprising subjecting-a polysulfoniurn salt insolution with said electrolysis solvent to an electrical potentialsufiicient to reduce said polysulfonium salt, said polysulfonium salthaving the formula wherein R and R are hydrocarbon orhydroxy-substituted hydrocarbon groups of 1 to 30 carbonatoms; n is aninteger and is at least 2; X is an n-valent hydrocarbon radical whosechain may be interrupted by oxygen, sulfur or nitrogen, or by a keto,ester or amide linkage; and

12 A is an electrolytically acceptable anion compatible with the solventmedium.

2. The process defined in claim 1 wherein said solvent I is water.

3. The process defined in claim 1 wherein said electroconductive articleis metallic.

4. The process defined in claim 3 wherein said electroconductive articleis at least one of iron, copper, zinc, lead, aluminum, magnesium,chromium, silver and tin; or a metal alloy consisting essentially of oneof the aforementioned metals.

5. The process defined by claim 4 wherein said electroconductivematerial is lead, iron, copper or an alloy thereof.

6. The process defined by claim 1 wherein R and R are hydrocarbon orhydroxy-substituted hydrocarbon groups of 1 to 4 carbon atoms.

7. The process defined by claim 1 wherein n is 2 and X is'cH2--C5H4CH2-,

wherein Yis alkylene, arylene or alkenylene of 1 to 10 carbon atoms,oxygen or sulfur.

8. The product produced by the process of claim 1.' 9. The productproduced by the process of claim 2.

References Cited UNITED STATES PATENTS 3,417,003 12/ 1968 Ross et al.204-481 3,448,127 6/ 1969 Dotzer 204-14 3,471,327 10/ 1969 Gerland, cta1. 204l81 3,477,924 11/ 1969 Gregorian 204l4 JOHN H. MACK, PrimaryExaminer T. TUFARIELLO, Assistant Examiner U.S. Cl. X.R.

2% with STATES PATENT QFFIQE QERTWICATE OF fiGECTiON Patent 3,697 DatedOctober 10 1972 Invent0r(s) Ritchie A. Wessling and William J. SettineriIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Col. '4, line 29 should read as follows:

-those of the formula CH -C H -QH Col. 7/8 approx. line 15, delete "n=lto 20" and insert ---n=2 to 20--' Sol. 10, line 33, delete"poly(p-xylene-o-diethy'lsulf d insert---poly(p-xylylene-oc-diethylsulfo a Col. 10, lines 35-39 correct theformula to read as follows:

(:1 (JH CH CH CH Col. 10, line 68 the last subscript of the formula nowappears as "l2or" Correct the subscript to read l or 2 Signed and sealedthis 18th day of December 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Y RENE D. TEGTMEYER Attesting Officer ActingCommissioner of Patents

